ASK THE EXPERTS: Off-Grid System Troubles

Intermediate
An off-grid home with a PV system.

We recently had an off-grid solar-electric system installed at our rural house, 2.5 hours west of Prince George, British Columbia, Canada. We had been running the home with a 3,000-watt Honda inverter generator. We have an electric fridge, water pump, TV, and LED lighting. Our “automatic” loads are the fridge and the pump.

We purchased a solar “package” that included twelve 340-watt Hanwha Q Cells PV modules, and a prebuilt, Magnum MS4448PAE inverter system with a solar charge controller. Our batteries are 24 used Surrette 2-KS-33PS lead-acid that were previously in service for eight months, according to our supplier. The system was installed in November 2017.

Our modules are only producing between 40 and 80 W each, even on sunny days. With fully charged batteries, we can only run loads for about 2.5 days. (However, when the batteries read at 100% SOC, the hydrometer says they are only at 1,220—and they have not come up. It takes more than 20 hours of running our 6.5 kW generator to fully charge the batteries. We paid more than $25,000 for the system, and our fuel costs have tripled compared to when we were running only the generator.

Our installers have offered multiple reasons why we are having problems, including “no sun.” But four of our neighbors have PV systems that work fine—in times of little sun, they run their generators about two hours a day, three days a week. Our installers have suggested we add another PV array to help charge the batteries. Our theory is that it’s the batteries. What do you think?

Dan & Liv Kelly • Burns Lake, BC, Canada

Off-grid systems are by far the most difficult to plan because there are so many factors to account for, including the average brightness of the sun at your location (insolation), shading, sun angle in winter versus summer, and your daily energy usage. First, verify the system design. Did the installer perform both a site evaluation and a load evaluation before designing the system? These are both critical to sizing a successful off-grid system.

The installer should have looked up historical solar data for your area, and used a shading assessment. With that data, a good estimate of the site’s solar potential is possible. From a solar insolation map, it appears that Burns Lake has a winter average of 1.0 to 1.5 full sun-hours. Without another RE source, some generator run time is necessary with so few sun-hours. Next, you should have completed a load evaluation spreadsheet to size your battery bank so that it ideally stays above 70% state of charge (SOC) for lead-acid batteries.

Although your battery bank can store about 84.8 kWh of energy, only half of that should be used, to avoid damaging the batteries from low SOC. That gives you 22.2 kWh of usable storage. Do the math to see whether your array—with its orientation and shading—can keep up with your usage and the battery losses.

Your PV charge controllers and your inverter/charger from the generator may not be programmed correctly. Be sure that the bulk, absorb, and float settings meet the battery manufacturer’s specifications. The absorb phase is especially important and often neglected—it is timed for two to four hours and lets the system store the last 10% to 20% of incoming energy to bring the SOC to 100%. If your incoming PV or generator power isn’t able to push the battery bank through the entire absorb cycle, chronic undercharging could result, which will eventually damage the battery bank.

An indication of this is your measurement of the specific gravity (1.22), which indicates about 70% to 80% SOC, when your battery monitor showed 100% SOC. Battery monitors are handy tools, but they don’t account well for absorb-phase charging. Specific gravity is the only reading that is always correct for measuring actual SOC.

It’s unlikely that either a bad battery bank or a miswired PV array is the cause of your problems, but it is possible. To check for this, look at the watts reading on the charge controller. Are those low numbers what is showing on the display?

Ask the installer to bring an I-V curve analyzer to your site to check the output of each string of PV modules, and measure while also measuring both solar irradiance (how bright the sun is) and array temperature. If a PV analyzer is not available, you could use a digital voltmeter and clamp-on ammeter to compare each string in the array. The addition of an irradiance meter and infrared thermometer would give you even more detailed performance information from each string. Have them bring a battery load tester to track its performance under load, while timing the process and performing specific-gravity tests.

I suspect that you are simply using more energy than your PV system is producing during winter. If the battery bank is sized correctly for your loads, these problems should disappear in the summer, with more sun-hours.

I always avoid prepackaged off-grid systems, and instead design exactly for the customer’s resources, loads, and budget. Off the grid, one size does not fit all, and I hope your system designer will make it right for you.

Dan Fink • Buckville Consulting

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